1. Field of the Invention
The present invention relates in general to Global Positioning System (GPS) receivers, and in particular to systems, methods, and apparatuses for reducing or eliminating auto-correlation or cross-correlation in weak Code Division Multiple Access (CDMA) signals in the presence of strong CDMA signals.
2. Description of the Related Art
Cellular telephony, including Personal Communication System (PCS) devices, has become commonplace. The use of such devices to provide voice, data, and other services, such as Internet access, has provided many conveniences to cellular system users. Further, other wireless communications systems, such as two-way paging, trunked radio, Specialized Mobile Radio (SMR) that is used by police, fire, and paramedic departments, have also become essential for mobile communications.
A current thrust in the cellular and PCS arena is the integration of Global Positioning System (GPS) technology into cellular telephone devices and other wireless transceivers. For example, U.S. Pat. No. 5,874,914, issued to Krasner, which is incorporated by reference herein, describes a method wherein the basestation (also known as the Mobile Telephone Switching Office (MTSO)) transmits GPS satellite information, including Doppler information, to a remote unit using a cellular data link, and computing pseudoranges to the in-view satellites without receiving or using satellite ephemeris information.
This current interest in integrating GPS with cellular telephony stems from a new Federal Communications Commission (FCC) requirement that cellular telephones be locatable within 50 feet once an emergency call, such as a “911” call (also referred to as “Enhanced 911” or “E911”) is placed by a given cellular telephone. Such position data assists police, paramedics, and other law enforcement and public service personnel, as well as other agencies that may need or have legal rights to determine the cellular telephone's position. Further, GPS data that is supplied to the mobile telephone can be used by the mobile telephone user for directions, location of other locations that the cellular user is trying to locate, determination of relative location of the cellular user to other landmarks, directions for the cellular user via Internet maps or other GPS mapping techniques, etc. Such data can be of use for other than E911 calls, and would be very useful for cellular and PCS subscribers.
The approach in Krasner, however, is limited by the number of data links that can be connected to a GPS-dedicated data supply warehouse. The system hardware would need to be upgraded to manage the additional requirements of delivering GPS information to each of the cellular or PCS users that are requesting or requiring GPS data, which requirements would be layered on top of the requirements to handle the normal voice and data traffic being managed and delivered by the wireless system.
Krasner, however, does not discuss the problems of acquisition of a GPS satellite signal in difficult environments, such as urban areas, or where the mobile receiver has a limited or completely blocked view of the satellites. Inherent in such difficult environments is the ability of a sensitive receiver to acquire spurious signals in the electromagnetic spectrum.
Some of these spurious signals emanate from the GPS satellite that the mobile receiver is trying to acquire. If the mobile receiver sweeps through a subset of all of the possible codes, and finds a signal that is above the noise floor, the receiver will lock onto this signal. However, the receiver has no way of knowing if the signal it has chosen to lock onto is the proper signal, especially in weak signal environments. This type of event, where the receiver locks onto a spurious signal emanating from the GPS satellite of interest, is called “auto-correlation.” Auto-correlation can also occur in a strong signal environment, where the signal acquired is not the proper signal.
Other spurious signals emanate from other GPS satellites that are either within the line of sight of the mobile receiver, or, because of multi-path conditions, is not within the line of sight of the mobile receiver, and create the same problems as auto-correlation scenarios described above. However, when the spurious signal emanates from a GPS satellite other than the satellite of interest, the event is called “cross-correlation.”
Currently, there are no methods or devices designed to determine whether an auto-correlation or cross-correlation event has occurred. There are also no methods or devices designed to correct such events to ensure that the receiver is locked onto the proper signal.
It can be seen, then, that there is a need in the art for a method to determine whether an auto-correlation event or cross-correlation event has occurred. It can also be seen that there is a need in the art for a method to correct auto-correlation or cross-correlation events to allow the GPS receiver to lock onto the proper signal. It can also be seen that there is a need in the art for an apparatus to determine whether an auto-correlation event or cross-correlation event has occurred. It can also be seen that there is a need in the art for an apparatus to correct auto-correlation or cross-correlation events to allow the GPS receiver to lock onto the proper signal.